Fixes for gnome-session 3.

Fixes for gnome-session 3.

We run by default with a panel (default gnome-panel) and then run Openbox
without any panel if one cannot be found.

- Adds a fallback session for if a panel (default gnome-panel) not found.
- Removes notifications as a requirement.  Seems to be a legacy thing judging
  from the ubuntu .session files.  Notifications are being provided by
  notify-osd on modern systems, and you can't check for its presence in
  gnome-session (gnome-classic.session always fails because it looks for it, so
  ubuntu falls back to gnome-fallback.session).